What indication do you have that the PB is better built? I own both and, if anything, the iBook is the more durable machine. It would take a lot more force to crack the polycarbonate shell of hte iBook then it does to dent the aluminum of the PB.

The keyboard is better on the PB - no doubt. The iBook's keyboard flexes more. I assume this is because it lifts up for access to the RAM/WiFi while the PB has a solid backing. But I don't see how you can extrapolate this one difference into an overall statement about build quality.

Like others have said, the biggest choice comes down to the screen. (As a side note I'm bitterly disappointed that today's iBook updates are so minor. I was fully prepared to go buy a higher res, widescreen iBook today.)